Negative Electrode for Lithium Ion Secondary Battery and Lithium Ion Secondary Battery

ABSTRACT

A negative electrode for a lithium ion secondary battery includes: a negative electrode current collector ( 11 ); and a negative electrode active material for a lithium ion secondary battery, which is disposed on the negative electrode current collector and contains a carbon material and an aqueous binder. The carbon material is a graphite particle having a covering layer containing amorphous carbon by 5 wt % or less relative to a total weight of the carbon material.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No.2016-128899 filed with the Japan Patent Office on Jun. 29, 2016, theentire content of which is hereby incorporated by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to a negative electrode used for anonaqueous electrolyte battery, particularly a lithium ion secondarybattery.

2. Description of the Related Art

Nonaqueous electrolyte batteries have been practically used as batteriesfor vehicles including hybrid vehicles, electric vehicles, and the like.Examples of the batteries used for the on-vehicle power source includelithium ion secondary batteries. Lithium ion secondary batteries havebeen required to have various characteristics including the outputcharacteristic, the energy density, the capacity, the lifetime, and thehigh-temperature stability. In particular, in order to improve theinput/output characteristic of the battery, various improvements for theelectrode have been attempted.

For example, JP-A-2001-229914 has suggested the carbon negativeelectrode for a secondary battery, which has the high capacity and inwhich the gas generation in the charging is suppressed, and thesecondary battery including this negative electrode. JP-A-2001-229914 isfeatured in that the active material for the negative electrode containsgraphite covered with amorphous carbon as below. In the graphite coveredwith amorphous carbon, the weight decrement in the first stage based ona predetermined measurement method obtained by the thermogravimetry (TG)is 3 to 20% of the weight before the temperature increase.

SUMMARY

A negative electrode for a lithium ion secondary battery includes: anegative electrode current collector; and a negative electrode activematerial for a lithium ion secondary battery, which is disposed on thenegative electrode current collector and contains a carbon material andan aqueous binder. The carbon material is a graphite particle having acovering layer containing amorphous carbon by 5 wt % or less relative toa total weight of the carbon material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a negativeelectrode for a lithium ion secondary battery according to oneembodiment of the present disclosure;

FIGS. 2A and 2B are schematic views illustrating a carbon material ofthe negative electrode for a lithium ion secondary battery according toone embodiment of the present disclosure; and

FIG. 3 is a schematic cross-sectional view illustrating a lithium ionsecondary battery according to one embodiment of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, for purpose of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the disclosed embodiments. It will be apparent,however, that one or more embodiments may be practiced without thesespecific details. In other instances, well-known structures and devicesare schematically shown in order to simplify the drawing.

In the secondary battery according to JP-A-2001-229914 in which thegraphite covered with amorphous carbon is used as the carbon material inthe negative electrode active material, the gas generation in thecharging can be suppressed. However, covering graphite with muchamorphous carbon makes it easier to separate the negative electrodeactive material layer from the current collector such as a metal foil onwhich the negative electrode active material layer has been formed.

In view of the above, it is an object of the present disclosure toimprove the durability of the negative electrode for a lithium ionsecondary battery by optimizing the ratio between amorphous carbon andgraphite in the carbon material used for the negative electrode activematerial.

A negative electrode for a lithium ion secondary battery according tothe present disclosure includes: a negative electrode current collector;and a negative electrode active material for a lithium ion secondarybattery, which is disposed on the negative electrode current collectorand contains a carbon material and an aqueous binder. The carbonmaterial is a graphite particle having a covering layer containingamorphous carbon by 5 wt % or less relative to a total weight of thecarbon material.

The negative electrode for a lithium ion secondary battery according toan embodiment of the present disclosure has the excellent durabilitybecause of having the high adhesive power between the negative electrodeactive material layer and the metal foil. The lithium ion secondarybattery including this negative electrode for a lithium ion secondarybattery has the excellent durability and the long lifetime.

An embodiment of the present disclosure will be described below. In thepresent embodiment, a lithium ion secondary battery includes a powergenerating element in a package. The power generating element includes apositive electrode, a negative electrode, a separator, and anelectrolyte solution. Here, the negative electrode for a lithium ionsecondary battery (hereinafter also referred to as “negative electrode”simply) includes a negative electrode current collector, and a negativeelectrode active material layer disposed on the negative electrodecurrent collector. Specifically, the negative electrode is a batterymember with a shape like a thin plate or a sheet and includes thenegative electrode current collector and the negative electrode activematerial layer. The negative electrode active material layer is formedby applying on the negative electrode current collector, a mixturecontaining a negative electrode active material, a binder, and ifnecessary, a conductive agent.

Here, the negative electrode active material preferably contains acarbon material. In particular, the carbon material is preferably agraphite particle having a covering layer of amorphous carbon of 5 wt %or less relative to the total weight of the carbon material. Here, “thegraphite particle having the covering layer of amorphous carbon” refersto the graphite particle having its surface covered with amorphouscarbon. “The graphite particle having the covering layer of amorphouscarbon” only needs to have a part of the surface of the graphiteparticle covered with amorphous carbon, and it is not necessary that theentire surface of the graphite particle is covered with amorphouscarbon. Moreover, “the covering layer” does not necessarily mean thelayer having the uniform thickness. In the present embodiment, it isimportant that the graphite particle is covered with amorphous carbon of5 wt % or less relative to the total weight of the carbon material.

When graphite (graphite particles) used in each embodiment is containedin the negative electrode active material layer, the output of thebattery can be improved even if the state of charge (SOC) of the batteryis low, and this is advantageous. Graphite is the hexagonal crystalcarbon material having the hexagonal-plate-like crystal structure, andis also referred to as black lead, graphite, or the like. The shape ofthe graphite is preferably like a particle.

As the graphite (graphite particles), there are natural graphite(natural graphite particles) and artificial graphite (artificialgraphite particles). Natural graphite is inexpensive and can be obtainedin large quantity, and moreover has the stable structure and theexcellent durability. Since the artificial graphite is the artificiallyproduced graphite and has high purity (the impurities such as allotropesare hardly contained), the artificial graphite has the low electricresistance. Either the natural graphite or the artificial graphite canbe used suitably as the carbon material in the present embodiment. Inparticular, the natural graphite with a covering layer of amorphouscarbon or the artificial graphite with a covering layer of amorphouscarbon is preferably used.

Note that amorphous carbon used in each embodiment may partially have astructure similar to that of graphite. Amorphous carbon is the carbonmaterial which includes microcrystals forming the network randomly andis amorphous as a whole. Examples of the amorphous carbon include carbonblack, coke, activated carbon, carbon fiber, hard carbon, soft carbon,and mesoporous carbon. The graphite particle with a covering layer ofamorphous carbon used in the embodiment may be either the naturalgraphite particle with a covering layer of amorphous carbon or theartificial graphite with a covering layer of amorphous carbon. Whenthese are used as the carbon material of the negative electrode activematerial, the decomposition of the electrolyte solution is suppressedand the negative electrode can have the higher durability. In addition,the gas generation in charging the battery is suppressed. For thisreason, the durability of the battery itself is improved.

Here, in the case of using artificial graphite as the graphite, theinterlayer distance d value (d₀₀₂) is preferably 0.337 nm or more. Thestructure of the crystal of the artificial graphite is generally thinnerthan that of natural graphite. In the case of using the negativeelectrode active material for a lithium ion secondary battery containingthe artificial graphite, it is necessary that the artificial graphitehas the interlayer distance at which the intercalation of lithium ionsis possible. The interlayer distance at which theintercalation/deintercalation of lithium ions is possible can beestimated based on the d value (d₀₀₂). If the d value is 0.337 nm ormore, the intercalation/deintercalation of lithium ions is possible.

The reason why the graphite particle with the covering layer containingamorphous carbon by 5 wt % or less relative to the total weight of thecarbon material is used as the carbon material is not because of theparticular theory but can be considered as below. FIG. 1 is a schematicview of the negative electrode in which the negative electrode activematerial layer is disposed on the surface of the negative electrodecurrent collector. As illustrated in FIG. 1, the negative electrode 1for a lithium ion secondary battery includes a negative electrodecurrent collector 101, a negative electrode active material 102, aconductive agent 103, and a binder 104. In the negative electrode 1 fora lithium ion secondary battery, a mixture of the negative electrodeactive material 102, the conductive agent 103, and the binder 104 isstacked on a surface of the negative electrode current collector 101,and this forms the negative electrode active material layer. In FIG. 1,the binder 104 binds the particles of the negative electrode activematerial 102 as well as the negative electrode active material layer andthe negative electrode current collector 101. Then, the conductive agent103 exists to fill the space between the particles of the negativeelectrode active material 102 to promote the transfer of the electrons.It is particularly preferable that the carbon material is used as thenegative electrode active material 102 in the negative electrode activematerial layer, carbon black is used as the conductive agent 103, and anaqueous binder is used as the binder 104. In the negative electrodeactive material 102, the particles having almost the same shape and sizeare in contact with each other adjacently and adhere to each otherthrough the binder 104.

Next, FIG. 2A and FIG. 2B schematically show the cross-section of thenegative electrode active material. The following description is thepresumption and the effect of the present embodiment is not limited tothe effect obtained by the following structure only. The negativeelectrode active material 102 contains graphite particles 106 with acovering layer containing amorphous carbon 105. The graphite particles106 have almost the same size but the shape is uneven and irregular.Here, if the graphite particles 106 are provided with the covering layercontaining amorphous carbon 105 by 5 wt % or less relative to the totalweight of the carbon material, the unevenness on the surface of thegraphite particles 106 is not covered up completely. Therefore, thegraphite particles 106 still have the uneven shape (FIG. 2A). For thisreason, the adjacent carbon materials can be in contact with each otherat a plurality of points. However, if the graphite particles 106 areprovided with the covering layer containing amorphous carbon 105 by morethan 5 wt % relative to the total weight of the carbon material, theunevenness on the surface of the graphite particles 106 is covered upcompletely. Therefore, most of the graphite particles 106 have thespherical or spheroidal shape (FIG. 2B). In this case, the adjacentcarbon materials are in contact with each other at one point only. Ifthe covering with amorphous carbon is too much, the carbon materials arein contact with each other at fewer points, so that the binding of thecarbon materials may become weak. On the other hand, if the coveringwith amorphous carbon is too little, the decomposition of theelectrolyte solution or the like may occur on the surface of thenegative electrode when the battery is charged. It is difficult tosuppress the gas generation because of this. In view of this, thebalance of the amount of amorphous carbon for the covering relative tothe total weight of the carbon material is examined. As a result, it hasbeen understood that the graphite particles having the covering layercontaining amorphous carbon by 5 wt % or less relative to the totalweight of the carbon material can suppress the separation of thenegative electrode active material layer while suppressing the gasgeneration in the charging. In this case, the covering layer containsamorphous carbon preferably by 0.5 wt % or more, more preferably by 1 wt% or more, relative to the total weight of the carbon material.

It is particularly preferable that the binder included in the negativeelectrode active material layer is aqueous binder. The binder plays therole of binding together the particles of the carbon material as thenegative electrode active material, and binding together the negativeelectrode active material layer and the metal foil. Examples of thepreferable aqueous binder include synthetic rubber such as styrenebutadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR),isoprene rubber (IR), and acrylonitrile butadiene rubber (NBR); andpolysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guargum, and pectin. In particular, using SBR, CMC, and a mixture thereof asthe aqueous binder can improve the adhesive power between the carbonmaterials.

Another example of the binder is polyvinylidene fluoride (PVDF). If PVDFis used as the binder, N-methylpyrrolidone (NMP) can be used as thesolvent instead of water. In this case, the gas generation due to theremaining moisture can be suppressed. Other examples of the binderinclude fluorine resins such as polytetrafluoroethylene (PTFE) andpolyvinyl fluoride (PVF), and conductive polymers such as polyanilines,polythiophenes, polyacetylenes, and polypyrroles, in addition to PVDF.

The binder is contained preferably by approximately 4 to 7 wt % relativeto the weight of the entire negative electrode active material layer.When the binder is contained in the above range, the binding force ofthe negative electrode material can be secured and the resistance of thenegative electrode can be maintained low.

The negative electrode active material layer may contain a conductiveagent if necessary. Examples of the conductive agent include carbonfiber such as carbon nanofiber, carbon black such as acetylene black andKetjen black, and other carbon materials such as activated carbon,mesoporous carbon, fullerenes, and carbon nanotube. In addition, thenegative electrode active material layer may contain an additivegenerally used for forming the electrode, such as thickener, dispersant,and stabilizer.

In the embodiment, the negative electrode active material layer can beformed as below. First, the carbon material as the negative electrodeactive material, a binder, and a conductive agent are mixed in anappropriate proportion in a solvent (such as water orN-methylpyrrolidone (hereinafter referred to as “NMP”)); thus, slurry isformed. Next, this slurry is applied or rolled on a negative electrodecurrent collector including a metal foil (such as a copper foil). Thesolvent is evaporated by heating the negative electrode currentcollector. Thus, the negative electrode active material layer can beformed. On this occasion, preferably, the slurry is disposed so that theweight of the negative electrode active material layer after theevaporation of the solvent is 2.5 to 10 mg/cm² on each surface of thenegative electrode current collector. The weight of the negativeelectrode active material layer on each surface of the negativeelectrode current collector can be adjusted by changing theconcentration of the slurry, the amount and the thickness of the slurry,the heating time to evaporate the solvent, and the like as appropriate.It is preferable to set the weight of the negative electrode activematerial layer to be small because the negative electrode will havelower resistance. However, it is very difficult to set the weight of thenegative electrode active material layer on one surface to be 2.5mg/cm². This is reason why the weight of the negative electrode activematerial layer is preferably set in the range of 2.5 to 10 mg/cm² oneach surface of the negative electrode current collector.

In the lithium ion secondary battery according to the embodiment, thepositive electrode is a battery member with a shape like a thin plate ora sheet. This member has a positive electrode active material layerformed by applying or rolling, and then drying a mixture of a positiveelectrode active material, a binder, and if necessary, a conductiveagent on a positive electrode current collector such as a metal foil.The positive electrode active material layer preferably contains thepositive electrode active material containing lithium nickel compositeoxide. The lithium nickel composite oxide is transition metal compositeoxide containing lithium and nickel, which is represented by the generalformula Li_(x)Ni_(y)Me_((1-y))O₂ (here, Me is at least one or more kindsof metals selected from the group consisting of Al, Mn, Na, Fe, Co, Cr,Cu, Zn, Ca, K, Mg, and Pb).

The positive electrode that can be used in the embodiment includes thepositive electrode where the positive electrode active material layercontaining the positive electrode active material is disposed on thepositive electrode current collector. Preferably, the positive electrodeactive material layer of the positive electrode is formed as below. Amixture containing the positive electrode active material, the binder,and the conductive agent added if necessary is applied or rolled on thepositive electrode current collector including a metal foil such as analuminum foil. After that, the drying step is performed and thus, thepositive electrode active material layer is obtained. In eachembodiment, the positive electrode active material layer preferablycontains the positive electrode active material containing lithiumnickel composite oxide. The lithium nickel composite oxide is thetransition metal composite oxide containing lithium and nickel, which isrepresented by the general formula Li_(x)Ni_(y)Me_((1-y))O₂ (here, Me isone or more kinds of metals selected from the group consisting of Al,Mn, Na, Fe, Co, Cr, Cu, Zn, Ca, K, Mg, and Pb).

The positive electrode active material layer may further contain thepositive electrode active material containing lithium manganesecomposite oxide. Examples of the lithium manganese composite oxideinclude lithium manganate (LiMnO₂) with a zig-zag-layered structure andspinel type lithium manganate (LiMn₂O₄). By using the lithium manganesecomposite oxide additionally, the positive electrode can be fabricatedat lower cost. It is particularly preferable to use the spinel typelithium manganate (LiMn₂O₄) because of having the excellent stability ofthe crystal structure in the overcharged state.

It is particularly preferable that the positive electrode activematerial layer contains the positive electrode active materialcontaining lithium nickel manganese cobalt composite oxide with alayered crystal structure represented by the general formulaLi_(x)Ni_(y)Co_(z)Mn_((1-y-z))O₂. Here, x in the general formula is anumber satisfying the relation of 1≦x≦1.2. In addition, y and z arepositive numbers satisfying y+z<1, and y is 0.5 or less. As moremanganese is contained, it becomes more difficult to form the compositeoxide with a single phase. Therefore, desirably, the relation of1−y−z≦0.4 is satisfied. Moreover, as more cobalt is contained, the costwill increase and moreover, the capacity will decrease. Therefore,desirably, the relations of z<y and z<1−y−z are satisfied. In order toobtain the high-capacity battery, it is desirable that the relations ofy>1−y−z and y>z and are satisfied.

Examples of the conductive agent used if necessary for the positiveelectrode active material layer include carbon fiber such as carbonnanofiber, carbon black such as acetylene black and Ketjen black, andother carbon materials such as activated carbon, graphite, mesoporouscarbon, fullerenes, and carbon nanotube. In addition, the positiveelectrode active material layer may appropriately contain an additivegenerally used for forming the electrode, such as thickener, dispersant,and stabilizer.

Examples of the binder used for the positive electrode active materiallayer include: fluorine resins such as polyvinylidene fluoride (PVDF),polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF); conductivepolymers such as polyanilines, polythiophenes, polyacetylenes, andpolypyrroles; synthetic rubber such as styrene butadiene rubber (SBR),butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR),and acrylonitrile butadiene rubber (NBR); and polysaccharides such ascarboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin.

In the lithium ion secondary battery according to the embodiment, theseparator is a film-shaped battery member. This member is to secure theconductivity of lithium ions between the negative electrode and thepositive electrode by separating the positive electrode and the negativeelectrode from each other. The separator used in the embodiment includesthe olefin resin layer. The olefin resin layer is a layer containingpolyolefin obtained by polymerizing or co-polymerizing α-olefin such asethylene, propylene, butene, pentene, or hexene. In the embodiment, theolefin resin layer is preferably a layer with a structure having poresclosed when the battery temperature has increased, i.e., a layercontaining the porous or microporous polyolefin. When the olefin resinlayer has such a structure, even if the battery temperature shouldincrease, the separator is closed (shutdown) to block the ion flow. Toachieve the shutdown effect, it is particularly preferable to use theporous polyethylene film. The separator may have a heat-resistantmicroparticle layer. In this case, the heat-resistant microparticlelayer is provided to prevent the stop of the battery function when thebattery generates heat. This heat-resistant microparticle layer containsstable and heat-resistant inorganic microparticles that can resisttemperatures of 150° C. or more and do not easily reactelectrochemically. Examples of such an inorganic microparticle includeinorganic oxide such as silica, alumina (α-alumina, β-alumina, andθ-alumina), iron oxide, titanium oxide, barium titanate, and zirconiumoxide, and minerals such as boehmite, zeolite, apatite, kaolin, spinel,mica, and mullite. The ceramic separator including the heat-resistantlayer can also be used.

In the lithium ion secondary battery according to the embodiment, theelectrolyte solution is a solution with the electric conductivity thatis obtained by dissolving the ionic substance in the solvent. In theembodiment, in particular, a nonaqueous electrolyte solution is used.The power generating element including the positive electrode, thenegative electrode, the separator, and the electrolyte solutionconstitutes one unit of the main components of the battery. The powergenerating element usually includes the stack including the positiveelectrode and the negative electrode which are stacked on each otherwith the separator interposed therebetween. In the lithium ion secondarybattery according to the present embodiment, this stack is impregnatedwith the electrolyte solution.

A preferable example of the electrolyte solution used in the embodimentaccording to the present disclosure is the nonaqueous electrolytesolution, and may be the mixture containing: a linear carbonate such asdimethyl carbonate (hereinafter referred to as “DMC”), diethyl carbonate(hereinafter referred to as “DEC”), ethylmethyl carbonate (hereinafterreferred to as “EMC”), di-n-propyl carbonate, di-i-propyl carbonate,di-n-butyl carbonate, di-i-butyl carbonate, or di-t-butyl carbonate; anda cyclic carbonate such as propylene carbonate (PC) or ethylenecarbonate (hereinafter referred to as “EC”). The electrolyte solution isobtained by dissolving a lithium salt such as lithiumhexafluorophosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), orlithium perchlorate (LiClO₄) in such a carbonate mixture.

The electrolyte solution preferably contains PC and EC corresponding tothe cyclic carbonate and DMC and EMC corresponding to the linearcarbonate. In particular, PC is the solvent with the low coagulatingpoint, and is useful for improving the output when the battery has thelow temperature. However, it is known that the compatibility of PC tothe graphite used as the negative electrode active material is a littlelow. EC is the solvent with the high polarity, i.e., the high dielectricconstant. EC is useful as the component of the electrolyte solution fora lithium ion secondary battery. However, EC has a high melting point(coagulating point) and is solid at room temperature. Therefore, even ifthe mixed solvent containing EC is prepared, the coagulation and thedeposition may occur at low temperature. DMC is the solvent with the lowviscosity and the high diffusion coefficient. However, DMC has the highmelting point (coagulating point). Therefore, it may happen that theelectrolyte solution is coagulated at low temperature. Like DMC, EMC isthe solvent with the low viscosity and the high diffusion coefficient.In this manner, the components of the electrolyte solution have thedifferent characteristics. In order to improve the output when thebattery has the low temperature, it is important to consider the balancebetween these components. By adjusting the ratio between the cycliccarbonate and the linear carbonate to be contained, the electrolytesolution having the low viscosity at room temperature and maintainingits property even at the low temperature can be obtained.

The electrolyte solution may contain the cyclic carbonate compound asthe additive. Examples of the cyclic carbonate used as the additiveinclude vinylene carbonate (VC). A cyclic carbonate compound with ahalogen as the additive may be used. These cyclic carbonates are thecompounds that form a protective film for the positive electrode and thenegative electrode in the process of charging and discharging thebattery. In particular, the cyclic carbonates can prevent thesulfur-containing compound such as the disulfonic acid compound or thedisulfonic acid ester compound from attacking the positive electrodeactive material containing the lithium nickel composite oxide. Examplesof the cyclic carbonate compounds with a halogen include fluoroethylenecarbonate (FEC), difluoroethylene carbonate, trifluoroethylenecarbonate, chloroethylene carbonate, dichloroethylene carbonate, andtrichloroethylene carbonate. Fluoroethylene carbonate corresponding tothe cyclic carbonate compound with a halogen and an unsaturated bond isparticularly preferably used.

The electrolyte solution may further contain a disulfonic acid compoundas the additive. The disulfonic acid compound is a compound having twosulfo groups in one molecule. The disulfonic acid compound incorporatesa disulfonate compound corresponding to a salt formed by the reactionbetween the sulfo group and the metal ion, and a disulfonic acid estercompound having the ester bond including the sulfo group. One or two ofthe sulfo groups of the disulfonic acid compound may react with themetal ion to form the salt or may be in the anion state. Examples of thedisulfonic acid compound include methanedisulfonic acid,1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid,1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonicacid, biphenyldisulfonic acid, salts thereof (such as lithiummethanedisulfonate and lithium 1,3-ethanedisulfonate), and anionsthereof (such as methanedisulfonic acid anion and 1,3-ethanedisulfonicacid anion). Other examples of the disulfonic acid compound include adisulfonic acid ester compound. Among these disulfonic acid esters,linear disulfonic acid esters of alkyl diester and aryl diester, such asmethanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonicacid, 1,4-butanedisulfonic acid, benzenedisulfonic acid,naphthalenedisulfonic acid, and biphenyldisulfonic acid, and cyclicdisulfonic acid esters such as methylene methanedisulfonate, ethylenemethanedisulfonate, and propylene methanedisulfonate are preferablyused. Methylene methanedisulfonate (hereinafter referred to as “MMDS”)is particularly preferable.

The package of the lithium ion secondary battery according to thepresent embodiment includes the power generating element. Preferably,the power generating element is sealed in the package. The sealing meansthat the power generating element is covered with the package materialso that the power generating element is not exposed to the external air.That is to say, the package has a bag-like shape that can seal the powergenerating element inside. As the package, an aluminum laminate can beused.

Here, a structure example of the lithium ion secondary battery accordingto the present embodiment is illustrated in FIG. 3. As illustrated inFIG. 3, a lithium ion secondary battery 10 includes, as main components,negative electrode current collectors 11, negative electrode activematerial layers 13, separators 17, positive electrode current collectors12, and positive electrode active material layers 15. In FIG. 1, thenegative electrode active material layer 13 is disposed on each ofopposite surfaces of the negative electrode current collectors 11. Oneach of the opposite surfaces of the positive electrode currentcollectors 12, the positive electrode active material layer 15 isdisposed. Alternatively, the active material layer can be disposed ononly one surface of each current collector. The negative electrodecurrent collector 11, the positive electrode current collector 12, thenegative electrode active material layer 13, the positive electrodeactive material layer 15, and the separator 17 constitute one batteryunit, i.e., the power generating element (a unit cell 19 in thedrawing). A plurality of such unit cells 19 is stacked with theseparator 17 interposed therebetween. Extension portions extending fromthe negative electrode current collectors 11 are collected and bondedonto a negative electrode lead 25. Extension portions extending from thepositive electrode current collectors 12 are collected and bonded onto apositive electrode lead 27. The positive electrode lead is preferably analuminum plate and the negative electrode lead is preferably a copperplate. These leads may be partly coated with another metal (such asnickel, tin, or solder) or a polymer material. The positive electrodelead and the negative electrode lead are welded to the positiveelectrode and the negative electrode, respectively. The batteryincluding the stacked unit cells is covered with a package 29 so thatthe welded negative electrode lead 25 and positive electrode lead 27 areled out of the battery. An electrolyte solution 31 is poured into thepackage 29. The package 29 has a shape obtained by heat-sealing theperiphery of the two stacks.

EXAMPLES <Preparation of Negative Electrode Active Material>

Pitch pulverized to have a mean particle diameter of 60 μm and sphericalgraphite with a mean particle diameter of 16 μm were mixed in a dryprocess with a weight ratio of 8:92. This mixture was calcined at 1000°C. for 12 hours in a nitrogen atmosphere. This calcined mixture waspulverized and sieved through a 400-mesh sieve; thus, a carbon materialwith a mean particle diameter of 17 μm (graphite having a covering layerof amorphous carbon) was obtained (Example 1).

A carbon material was obtained by the same method as that of Example 1except that the mixing ratio between the pitch and the sphericalgraphite was set to 6:94 (Example 2).

A carbon material was obtained by the same method as that of Example 1except that the mixing ratio between the pitch and the sphericalgraphite was set to 5:95 (Example 3).

A carbon material was obtained by the same method as that of Example 1except that the mixing ratio between the pitch and the sphericalgraphite was set to 3:97 (Example 4).

A carbon material was obtained by the same method as that of Example 1except that the mixing ratio between the pitch and the sphericalgraphite was set to 2:98 (Example 5).

A carbon material was obtained by the same method as that of Example 1except that the mixing ratio between the pitch and the sphericalgraphite was set to 10:90 (Comparative Example 1).

A carbon material was obtained by the same method as that of Example 1except that the mixing ratio between the pitch and the sphericalgraphite was set to 15:85 (Comparative Example 2).

<Estimation of Amount of Amorphous Carbon for Covering>

The thermogravimetry (TG measurement) was performed on the carbonmaterials according to the above Examples and Comparative Examples inthe atmosphere at a temperature increasing speed of 5° C./min using thedifferential scanning calorimeter TG8121 (Rigaku Corporation). Theweight was plotted as the function of the temperature. The weightdecrease ratio until the point where the temperature differential curveΔTG of the weight change between 550° C. and 650° C. became the minimumwas used as the amount of amorphous carbon for the covering.

<Fabrication of Negative Electrode>

As the negative electrode active material, each of the above carbonmaterials was used. Carbon black powder (hereinafter “CB”) (Super-C65,manufactured by IMERYS GC) with a BET specific surface area of 62 m²/gas the conductive agent, and carboxymethyl cellulose (hereinafter “CMC”)and styrene butadiene copolymer latex (hereinafter “SBR”) as the binderresin were mixed in a solid content mass ratio ofCB:CMC:SBR=0.3:1.0:2.0. The above carbon material and the obtainedmixture were mixed at a ratio of 96.7:3.3. The resulting mixture wasadded to ion exchanged water and stirred, so that the materials wereuniformly mixed and dispersed; thus, slurry was prepared. The obtainedslurry was applied on a 10-μm-thick copper foil serving as the negativeelectrode current collector so that the dried slurry had a weight of 10mg/cm² on each surface. Next, the electrode was heated at 100° C. for 10minutes, so that water was evaporated. Thus, the negative electrodeactive material layer was formed. In addition, the electrode waspressed; thus, the negative electrode having the 67-μm-thick negativeelectrode active material layer applied on one surface of the negativeelectrode current collector was fabricated.

<Fabrication of Positive Electrode>

Lithium nickel composite oxide (lithium nickel cobalt manganese oxide(“NCM523”, i.e., nickel:cobalt:manganese=5:2:3, lithium/metal (exceptlithium) ratio=1.04, BET specific surface area: 62 m²/g)) with a meanparticle diameter D50 of 9 μm and lithium manganese oxide (LiMn₂O₄) weremixed at a ratio of 75:25 and the mixed oxide was obtained. This mixedoxide was used as the positive electrode active material. CB (Super-C65,manufactured by IMERYS GC) with a BET specific surface area of 62 m²/gand graphite powder (hereinafter “GR”) with a BET specific surface areaof 22 m²/g which correspond to the conductive agent, and PVDF (#7200,Kureha Battery Materials Japan Co., Ltd.) which corresponds to thebinder resin were mixed so that the solid content mass ratio wasCB:GR:PVDF=3:1:3. The mixed oxide and the mixture containing CB, GR, andPVDF were mixed at a ratio of 93:7, and then added to NMP as thesolvent. To this mixture, oxalic acid anhydrous (molecular weight: 90)as the organic moisture scavenger was added. The amount of added oxalicacid anhydrous was 0.03 parts by mass relative to 100 parts by mass ofthe solid content of the mixture excluding NMP. In addition, the mixturewith oxalic acid anhydrous added was dispersed and mixed for 30 minutesby a planetary method. Thus, the slurry having the above materialsuniformly dispersed was prepared. The obtained slurry was applied on a20-μm-thick aluminum foil serving as the positive electrode currentcollector. Next, the electrode was heated at 125° C. for 10 minutes toevaporate NMP; thus, the positive electrode active material layer wasformed. In addition, by pressing the electrode, the positive electrodewith the 80-μm-thick positive electrode active material layer applied onone surface of the positive electrode current collector was fabricated.

<Separator>

A 25-μm-thick ceramic separator which contained polypropylene and aheat-resistant microparticle layer containing alumina as theheat-resistant microparticles was used.

<Electrolyte Solution>

Ethylene carbonate (EC), diethyl carbonate (DEC), and ethylmethylcarbonate (EMC) were mixed at a ratio of 30:60:10 (volume ratio). Intothe obtained mixed nonaqueous solvent, lithium hexafluorophosphate(LiPF₆) as the electrolyte salt was dissolved so that the concentrationthereof became 0.9 mol/L. To the obtained mixed nonaqueous solvent,MMDS, VC, and FEC as the additives were dissolved so that the totalconcentration thereof became 1 wt %; thus, this mixed nonaqueous solventwas used as the electrolyte solution.

<Fabrication of Lithium Ion Secondary Battery>

From each of the positive electrode plate and the negative electrodeplate fabricated as above, rectangular members were cut into apredetermined size. On a part where coating was not applied forconnecting a positive electrode terminal, a positive electrode leadterminal made of aluminum was welded with ultrasonic waves. Similarly,on a part where coating was not applied for connecting a negativeelectrode terminal, a negative electrode lead terminal made of nickelwas welded with ultrasonic waves. The negative electrode plate and thepositive electrode plate were disposed on both surfaces of thepolypropylene porous separator in a state that both active materiallayers are overlapped on each other through the separator; thus, theelectrode stack was obtained. This electrode stack was wrapped with twoaluminum laminate films, and except one long side of the two aluminumlaminate films, the other three sides were attached throughheat-sealing. The electrolyte solution was poured into the laminatedfilm package with a liquid amount of 140%, so that the pores of theelectrode stack and the separator were impregnated with the electrolytesolution in vacuum. Next, the opening was heat-sealed under reducedpressure. Thus, a stacked lithium ion battery was obtained. After thisstacked lithium ion battery was charged for the first time, aging wasperformed at 45° C. for several days. Thus, the fabrication of thestacked lithium ion secondary battery was completed.

<Peeling Strength>

The 90°-peeling strength was measured by the method substantially basedon the testing methods of pressure-sensitive adhesive tapes and sheets(JISZ0237). A stainless-steel test piece and a copper foil side of eachnegative electrode with a width of 11 mm and a length of 10 cm wereattached together with a double-sided tape. Next, a tape as the backingmember was attached to the negative electrode active material layerside. Thus, a peeling strength test plate was formed. Each test platewas attached to the 90°-peeling test jig. One end of the test plate wasattached to a load cell (ZTS-2N digital force gauge, IMADA CO., LTD.).With the use of a vertical motorized test stand (MX2-500N, IMADA CO.,LTD.), the test plate was pulled up at an angle of 90° relative to thetest plate at a speed of 5 mm/s. As a result, the peeling occurredbetween the copper foil and the negative electrode active material layerof the negative electrode. The peeling force for approximately 15 mmafter the start of the pulling up of the test plate was ignored. Afterthat, the average peeling force in the region where the values of theload cell became stable was used as the peeling strength between thecopper foil and the negative electrode active material layer.

<Initial Charging-Discharging>

The initial charging-discharging was performed using the stacked lithiumion secondary battery fabricated as above. In the initialcharging-discharging, constant-current and constant-voltage (CC-CV)charging was performed at an atmospheric temperature of 25° C. with acurrent of 10 mA and an upper-limit voltage of 4.2 V. After that, agingwas performed at 45° C. for several days. Next, the constant-currentdischarging was performed with a current of 20 mA and an upper-limitvoltage of 2.5 V.

<Cycle Characteristic Test>

A cycle characteristic test was performed on the stacked lithium ionsecondary battery after the above initial charging-discharging. In thistest, one charging-discharging cycle is carried out under the 25°C.-environment, and this test includes constant-current constant-voltagecharging with a current of 100 mA, an upper-limit voltage of 4.15 V, andan ending current of 1 mA, and constant-current discharging with acurrent of 100 mA, ending at a lower-limit voltage of 2.5 V. Thischarging-discharging cycle was repeated 500 cycles (500 times). On thisoccasion, using the discharging capacity in the first cycle and thedischarging capacity in the 500-th cycle obtained from the measurement,the retention (%) of the discharging capacity in the 500-th cyclerelative to the discharging capacity in the first cycle (=thedischarging capacity in the 500-th cycle/the discharging capacity in thefirst cycle) was calculated. The calculation result was used as thereference of the durability of the battery.

Table 1 shows the peeling strength of the negative electrodes accordingto Examples 1 to 5 and Comparative Examples 1 and 2 described above, andthe cycle characteristic of the stacked lithium ion secondary batteriesincluding those negative electrodes. Note that the cycle characteristicis the relative value when the cycle characteristic measured inComparative Example 1 is 100.

TABLE 1 Amorphous carbon Peeling coverage strength Cycle Pitch:graphite(%) (mN/mm) characteristic Example 1 8:92 5 35 160 Example 2 6:94 4 38180 Example 3 5:95 3 42 200 Example 4 3:97 2 44 210 Example 5 2:98 1 50220 Comparative 10:90  6 25 100 Example 1 Comparative 15:85  10 10 70Example 2

The negative electrode according to the present disclosure has the highpeeling strength. In particular, the negative electrode with anamorphous carbon coverage of 1% (Example 5) has the remarkably improvedpeeling strength, and has the increased durability. The batteryincluding this negative electrode has the excellent cycle characteristicand therefore, it is understood that by adjusting the amount ofamorphous carbon that covers the negative electrode carbon material, thedurability of the negative electrode and the lifetime of the batteryitself can be improved. The battery including the negative electrodesatisfying the range of the present disclosure has the high output andhas the output capacity ratio suitable for the purpose as the on-vehiclebattery.

Examples of the present disclosure have been described so far butExamples merely show examples of the embodiment of the presentdisclosure. Limiting the technical range of the present disclosure tothe particular embodiment or the specific structure is not intended byExamples.

The negative electrode for a lithium ion secondary battery according tothe embodiment of the present disclosure may be any one of the followingfirst to third negative electrodes for a lithium ion secondary battery,and the lithium ion secondary battery according to the embodiment of thepresent disclosure may be the following first lithium ion secondarybattery.

The first negative electrode for a lithium ion secondary battery is anegative electrode for a lithium ion secondary battery in which anegative electrode active material for a lithium ion secondary batterycontaining a carbon material and an aqueous binder is disposed on anegative electrode current collector. The carbon material is a graphiteparticle that has a covering layer containing amorphous carbon by 5% orless relative to the total weight of the carbon material.

The second negative electrode for a lithium ion secondary battery is thefirst negative electrode for a lithium ion secondary battery, in whichthe aqueous binder is selected from styrene butadiene rubber,carboxymethyl cellulose, and a mixture thereof.

The third negative electrode for a lithium ion secondary battery is thefirst or second negative electrode for a lithium ion secondary battery,in which the graphite is natural graphite.

The first lithium ion secondary battery is a lithium ion secondarybattery including a power generating element including a positiveelectrode, a negative electrode, a separator, and an electrolytesolution inside a package. The negative electrode is any one ofabove-described first to third negative electrode for a lithium ionsecondary battery.

The foregoing detailed description has been presented for the purposesof illustration and description. Many modifications and variations arepossible in light of the above teaching. It is not intended to beexhaustive or to limit the subject matter described herein to theprecise form disclosed. Although the subject matter has been describedin language specific to structural features and/or methodological acts,it is to be understood that the subject matter defined in the appendedclaims is not necessarily limited to the specific features or actsdescribed above. Rather, the specific features and acts described aboveare disclosed as example forms of implementing the claims appendedhereto.

What is claimed is:
 1. A negative electrode for a lithium ion secondarybattery, comprising: a negative electrode current collector; and anegative electrode active material for a lithium ion secondary battery,which is disposed on the negative electrode current collector andcontains a carbon material and an aqueous binder, wherein the carbonmaterial is a graphite particle having a covering layer containingamorphous carbon by 5 wt % or less relative to a total weight of thecarbon material.
 2. The negative electrode for a lithium ion secondarybattery according to claim 1, wherein the aqueous binder is selectedfrom styrene butadiene rubber, carboxymethyl cellulose, and a mixturethereof.
 3. The negative electrode for a lithium ion secondary batteryaccording to claim 1, wherein the graphite particle is a particle ofnatural graphite.
 4. A lithium ion secondary battery comprising a powergenerating element inside a package, the power generating elementcomprising: a positive electrode; the negative electrode according toclaim 1; a separator; and an electrolyte solution.